Agents currently used to treat HIV infection attempt to block replication of the HIV virus by blocking HIV reverse transcriptase or by blocking HIV protease. Three categories of anti-retroviral agents in clinical use are nucleoside analogs (such as AZT), protease inhibitors (such as nelfinavir), and the recently introduced non-nucleoside reverse transcriptase inhibitors (NNI), such as nevirapine.
The recent development of potent combination anti-retroviral regimens has significantly improved prognosis for persons with HIV and AIDS. Combination therapies may be a significant factor in the dramatic decrease in deaths from AIDS (decrease in death rate as well as absolute number). The most commonly used combinations include two nucleoside analogs with or without a protease inhibitor.
Nevirapine is currently the only NNI compound which has been used in combination with AZT and/or protease inhibitors for the treatment of HIV. A new series of effective drug cocktails will most likely involve other NNIs in combination with nucleoside and protease inhibitors as a triple action treatment to combat the growing problem of drug resistance encountered in single drug treatment strategies.
The high replication rate of the virus unfortunately leads to genetic variants (mutants), especially when selective pressure is introduced in the form of drug treatment. These mutants are resistant to the anti-viral agents previously administered to the patient. Switching agents or using combination therapies may decrease or delay resistance, but because viral replication is not completely suppressed in single drug treatment or even with a two drug combination, drug-resistant viral strains ultimately emerge. Triple drug combinations employing one (or two) nucleoside analogs and two (or one) NNI targeting RT provide a very promising therapy to overcome the drug resistance problem. RT mutant strains resistant to such a triple action drug combination would most likely not be able to function.
Dozens of mutant strains have been characterized as resistant to NNI compounds, including L1001, K103N, V106A, E138K, Y118C and Y188H. In particular, the Y181C and K103N mutants may be the most difficult to treat, because they are resistant to most of the NNI compounds that have been examined.
Recently, a proposed strategy using a knock-out concentration of NNI demonstrated very promising results. The key idea in this strategy is to administer a high concentration of NNI in the very beginning stages of treatment to reduce the virus to undetectable levels in order to prevent the emergence of drug-resistant strains. The ideal NNI compound for optimal use in this strategy and in a triple action combination must meet three criteria:
1) very low cytotoxicity so it can be applied in high doses; PA1 2) very high potency so it can completely shut down viral replication machinery before the virus has time to develop resistant mutant strains; and PA1 3) robust anti-viral activity against current clinically observed drug resistant mutant strains. PA1 1. very low cytotoxicity; PA1 2. very high potency; and PA1 3. potent activity against at least one clinically observed drug resistant mutant strain.
Novel NNI designs able to reduce RT inhibition to subnanomolar concentrations with improved robustness against the most commonly observed mutants and preferably able to inhibit the most troublesome mutants are urgently needed. New antiviral drugs will ideally have the following desired characteristics: (1) potent inhibition of RT; (2) minimum cytotoxicity; and (3) improved ability to inhibit known, drug resistant strains of HIV. Currently, few anti-HIV agents possess all of these desired properties.
Two non-nucleoside inhibitors (NNI) of HIV RT that have been approved by the U.S. Food and Drug Administration for licensing and sale in the United States are nevirapine (dipyridodiazepinone derivative) and delavirdine (bis(heteroaryl)piperazine (BHAP) derivative, BHAP U-90152). Other promising new non-nucleoside inhibitors (NNIs) that have been developed to inhibit HIV RT include dihydroalkoxybenzyloxopyrimidine (DABO) derivatives, 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT) derivatives, PHETrahydrobenzondiazepine (TIBO), 2', 5'-Bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxathiole-2 ",2'-dioxide)pyrimidine (TSAO), oxathiin carboxanilide derivatives, quinoxaline derivatives, thiadiazole derivatives, and phenethylthiazolylthiourea (PETT) derivatives.
NNIs have been found to bind to a specific allosteric site of HIV- RT near the polymerase site and interfere with reverse transcription by altering either the conformation or mobility of RT, thereby leading to a noncompetitive inhibition of the enzyme (Kohlstaedt, L. A. et al., Science, 1992, 256, 1783-1790).
A number of crystal structures of RT complexed with NNIs have been reported (including .alpha.-APA, TIBO, Nevirapine, and HEPT derivatives), and such structural information provides the basis for further derivatization of NNI aimed at maximizing binding affinity to RT. However, the number of available crystal structures of RT NNI complexes is limited.
Given the lack of structural information, alternate design procedures must be relied upon for preparing active inhibitors such as PETT and DABO derivatives. One of the first reported strategies for systematic synthesis of PETT derivatives was the analysis of structure-activity relationships independent of the structural properties of RT and led to the development of some PETT derivatives with significant anti-HIV activity (Bell, F. W. et al., J. Med. Chem., 1995, 38, 4929-4936; Cantrell, A. S. et al., J. Med. Chem., 1996, 39, 4261-4274).
A series of selected phenethylthiazolylthiourea (PETT) derivatives targeting the NNI binding site of HIV reverse transcriptase (RT) were synthesized and tested for anti-human immunodeficiency virus (HIV) activity. The structure based design and synthesis of these PETT derivatives were aided by biological assays and their anti-HIV activity. Some of these novel derivatives were more active than AZT or Troviridine and abrogated HIV replication at nanomolar concentrations without any evidence of cytotoxicity. These compounds are useful in the treatment of HIV infection, and have particular efficacy against mutant strains, making them useful in the treatment of multi-drug resistant HIV.